Known in the prior art for impact absorption to protect the occupants of vehicles are essentially three types of devices:
1. Gas generators based on pure combustion of a generally solid pyrotechnic compound such as described in U.S. Pat. No. 3,865,660 and FR 2 730 965. The combustion of this type of propellant generates dust particles which are harmful to the airbag and to the comfort and health of the vehicle occupant(s). To improve the operation of such devices equipped with a solid propellant, EP 0 480 085 and EP 0 728 630 propose placing on the surface of the solid propellant bands of ignition pyrotechnic materials. On the other hand, EP 0 420 726 uses a propellant based on nitrocellulose that doesn't generate dust particles, but rather carbon monoxide, an inflammable toxic gas. It is also known, notably from application no. FR 2 682 374, that the decomposition products of a propellant of the double-base type can be oxidized by potassium perchlorate. However, as in the previously cited cases, the disadvantages of such a configuration are in particular to drastically raise the temperature of the gas and to generate very hot dust particles.
2. Gas generators based on combustion of a pyrotechnic compound provide for the heating of a neutral gas stored under pressure, referred to as hybrid generators such as the one described in U.S. Pat. No. 5,022,674. As in the preceding cases, the combustion of the pyrotechnic compound generates a large quantity of dust.
3. Hybrid gas generators whose products of the combustion of the pyrotechnic compound react with the oxygen stored in the compressed gas, referred to as reactive hybrid generators, such as the one described in application no. EP 0 673 809. In this case, the use of a propellant based on nitrocellulose or LOVA-type powder makes it possible to generate dust-free gas. However, combustion of the reaction products creates an excessive pressure in the structure of the gas generator which consequently must be reinforced.
The device corresponding to application no. GB 2,292,788 performs combustion downstream of the stored-gas chamber. However, since the post-combustion gas flows are not controlled, the characteristics of the generated gas (flow rate, composition, temperature) are not constant during deployment of the system and, thus, are not optimized. This technology allows use of other types of combustible compounds such as magnesium on an organic substrate as cited in U.S. Pat. No. 5,655,790. In this case, the generation of reducing entities is not regulated as in the case of propellants and functioning is similar to that of gas generators constituted of combustible gaseous mixtures the functioning of which corresponds to emptying a reservoir under very high pressure. Such examples are moreover described as examples in U.S. Pat. Nos. 5,460,406, 5,897,136 and EP 0 978 423.
Numerous studies have been carried out with the objective of formulating propellants, the oxidant source of which is ammonium nitrate. U.S. Pat No. 6,123,790 is an example. These propellants have the drawbacks of being difficult to ignite, having slow combustion rates at moderate pressures, of being hygroscopic and exhibiting aging of their mechanical properties which is difficult to control. With the same objective, U.S. Pat. No. 5,868,424, WO 00/32447 and WO 00/46079 describe pyrotechnic charges constituted of ammonium nitrates and propellant grains based on nitrocellulose or of the LOVA type. In this case, the combustion of the pyrotechnic charge requires a strong confinement which is obtained either in an expeller or in a high-pressure tube. In both cases, as soon as the pressure markedly decreases, the combustion rate is annihilated.
Also known in the prior art for particular applications are devices for deployment of a safety element that use compressed gas sources.
The major drawback of the devices for deployment of a safety element of the prior art that use compressed gas sources is the fact that this source must be confined in a reinforced container which is permanently subjected to overpressure until the deployment of the safety element.